The hypothalamus plays critical roles in homeostasis and energy metabolism, and thus defects in hypothalamic development result in serious pathological conditions, including diabetes and obesity (1). However, how the diverse array of hypothalamic cell types is specified and organized remains poorly defined, partly due to the anatomical complexity of this essential brain region. The central theme of this proposal is to address this critical issue with a focus on two neuronal subtypes in the hypothalamic arcuate nucleus (ARC), AgRP- and POMC- neurons, which directly sense nutrient and hormonal signals of energy availability from the periphery and relay this information to other neurons in the centra nervous system (CNS). The key function of AgRP-neurons is to respond to energy scarcity conditions, accomplished by increased expression of the orexigenic agouti-related peptide (AgRP) and neuropeptide Y (NPY) that stimulate feeding behavior and reduce energy expenditure. In contrast, POMC-neurons respond to conditions of energy abundance by suppressing feeding and increasing energy expenditure primarily via the upregulation of ?-melanocyte-stimulating hormone (?MSH), a proteolytic product of pro-opiomelanocortin (POMC) (1). Islet-1 (Isl1), a member of the LIM-homeodomain (LIM-HD) transcription factor family, plays crucial roles in determining cell fates during the development of several organs, such as spinal motor neurons and endocrine pancreatic islet cells (x). Expression of Isl1 has been reported in several regions of the hypothalamus (xx, xx). We also discovered that Isl1 is highly expressed in the ARC of both embryonic and postnatal mouse brains. Our analyses of conditional Isl1-null mice revealed that deletion of Isl1 in the hypothalamus results in postnatal death, associated with drastically reduced body size and abrogation in the expression of AgRP, NPY and POMC by ARC neurons. We also found critical roles of Isl1 in postnatal metabolic function of AgRP-neurons. Based on these seminal findings, we will test the hypothesis that Isl1 acts as a critical dual regulator of 1) the specification/development and 2) the postnatal function of AgRP- and POMC-neurons by coordinating a cohort of genes that establish the identity, maturation and energy homeostasis action of these neurons in developing and postnatal hypothalamus. We will test this hypothesis in the following three specific aims using an ensemble of cellular, biochemical, genetic, and unbiased genome-wide approaches. In the first aim, we wish to determine the temporal and spatial pattern of Isl1 expression in the developing and postnatal mouse hypothalamus. In the second aim, we will define the roles of Isl1 in the development and the energy balance function of AgRP- and POMC-neurons. In the final aim, we will define the target genes of Isl1 and its partner transcription factors that direct the developmental specification, maturation and function of the arcuate AgRP- and POMC-neurons.